A liquid crystal display panel has been widely used for, for example, television sets, personal computers, and display sections of other office automation equipment thanks to its thinner structure and lighter weight as compared with a display device using a cathode-ray tube, etc.
As a display system of the liquid crystal display panel, the twisted nematic(TN)type has been conventionally used in many cases. In the TN type, nematic liquid crystal is sealed into a gap between transparent electrode substrates, and liquid crystal molecules are disposed between the two substrates with each of the major axes being twisted by 90.degree. in succession, and a vertical electric field driving system, which carries out an image display by driving the liquid crystal molecules using an electric field arranged vertical to the transparent electrode substrate. However, in the above-mentioned liquid crystal display panel which uses the vertical electric field driving system such as the above-mentioned TN type, the actual anisotropy of refractive index varies for each viewing angle. Namely, this system has high dependency upon the viewing angle.
Here, a horizontal electric field driving system has been developed in earnest for improving viewing angle property, that is, for realizing a wide viewing angle in response to recent needs for a large-size display, etc. The horizontal electric field driving system is a system in which liquid crystal molecules are rotated by using a horizontal electric field in the in-plane direction to the substrate so as to provide an image display. Since the major axis of the liquid crystal molecule is always arranged in parallel with the substrate, the optical property inherently does not vary for each viewing angle.
Japanese Laid-Open Patent Publication No.36058/1995 (Tokukaihei 7-36058) discloses some kinds of electrode structures which use the horizontal electric field driving system. Referring to figures, the following explanation describes one example of a conventional liquid crystal display panel disclosed in the aforementioned Patent Publication which uses the horizontal electric field driving system.
FIG. 7 is a top view illustrating one pixel of the conventional liquid crystal display panel which uses the horizontal electric field driving system. This liquid crystal display panel is arranged in a manner so as to have liquid crystal sealed into a gap between two insulating substrates(not shown) opposing each other.
In FIG. 7, on a first insulating substrate (not shown), gate lines 91 and a common line 92 are formed so as to be in parallel with each other. A source line 93 is provided in the direction orthogonal to the gate line 91 and the common line 92. A thin film transistor (TFT) 94 is provided on the gate line 91, and a pixel electrode 95, which is connected with the source line 93 via the TFT 94, is arranged in parallel with the source line 93. Further, in the vicinity of a source line 93' which is disposed on the other end of the pixel, a common electrode 96, which branches out from the common line 92, is arranged in parallel with the source line 93'. Incidentally, an area which is surrounded by a pair of the gate lines 91 and a pair of the source lines 93 and 93' corresponds to one pixel.
These electrode wires are coated with an alignment film(not shown) which is provided on the insulating substrate. This substrate is arranged so as to oppose a second insulating substrate which is provided with an alignment film on the surface thereof in the same manner as the first insulating substrate. Liquid crystal is sealed between these two substrates. Further, this liquid crystal is subjected to an optical modulation in a display section 97 disposed between the pixel electrode 95 and the common electrode 96 which are arranged in parallel with each other.
Moreover, the alignment film is subjected to an aligning operation by using a rubbing method, etc. The aligning operation differs depending upon the dielectric-constant anisotropy of the liquid crystal molecule to be used. For example, in the case when a liquid crystal molecule 98 with positive dielectric-constant anisotropy is driven, as shown in FIG. 7, an initial aligning direction of a liquid crystal molecule 98 is arranged virtually in parallel with both the pixel electrode 95 and the common electrode 96, and is arranged so as to be inclined somewhat clockwise when viewed from the side of the second insulating substrate. Moreover, as shown in FIG. 7, each broken line passing one end of the liquid crystal molecule 98 indicates the direction in parallel with the pixel electrode 95 and the common electrode 96. With this arrangement, when voltage is applied between the pixel electrode 95 and the common electrode 96, the liquid crystal molecule 98 rotates so as to achieve the optical modulation.
However, in and around the display section 97, the gate line 91, the common line 92, and the source lines 93 and 93' are provided in addition to the pixel electrode 95 and the common electrode 96 which apply voltage for driving the liquid crystal molecule 98; therefore, it is very difficult to apply a uniform horizontal electric field to the substrate in the in-plane direction. The reason why is that voltage is applied to each of the electrodes and wires that are disposed in the vicinity of the display section 97 so that electric fields newly appear and affect one another.
The following explanation describes the relationship between the state of electric lines of force in the above-mentioned uneven horizontal electric fields and the aligning directions of the liquid crystal molecule.
As shown FIG. 7, during a writing period, to the gate line 91 is inputted a signal with electric potential which is relatively positive based on the electric potential of the common line 92 and the common electrode 96; meanwhile, electric potential, which is relatively negative, is applied during the other period(holding period).
Therefore, in the case when the pixel electrode 95 is applied a signal with electric potential which is relatively positive based on the electric potential of the common line 92 and the common electrode 96, during the holding period, as shown in the model of FIG. 7, an electric line of force 99 extends from the pixel electrode 95 in any one of directions of the common electrode 96, the common line 92, or the gate line 91.
Incidentally, in the case of the liquid crystal molecule with positive dielectric constant anisotropy, upon applying voltage, torque is exerted in the major axis direction of the liquid crystal molecule along the electric line of force.
Namely, as shown in FIG. 7, in the case of the liquid crystal molecule 98 with positive dielectric constant anisotropy, upon applying voltage, torque is exerted to in the major axis direction of the liquid crystal molecule 98 along the electric line of force 99. Therefore, as shown in FIG. 7, in an area A in which the electric line of force 99 virtually orthogonal to the pixel electrode 95 and the common electrode 96, during the holding period, when to the pixel electrode 95 is applied a signal with electric potential which is relatively positive based on the electric potential of the common line 92 and the common electrode 96, a liquid crystal molecule 98a, which exists close to the pixel electrode 95 in the area A, rotates in the direction of an arrow(to the right) in accordance with the initial aligning direction shown in FIG. 7. At this time, in an area B of the same pixel as well, when voltage is applied, a liquid crystal molecule 98b rotates in the direction of an arrow(to the right) in accordance with the initial aligning direction; however, in an area C, a liquid crystal molecule 98c rotates in the direction of an arrow(to the left) with regard to the initial aligning direction. In other words, in one pixel, some of the liquid crystal molecules 98 rotate to the opposite direction. Such a phenomenon is called a reverse twist. Further, a boundary, which appears between the area in which the liquid crystal molecule 98 rotates in the clockwise direction and the area in which the liquid crystal molecule 98 rotates in the counterclockwise direction, is called a disclination line. The disclination line is caused by the reverse twist.
As described above, referring to FIG. 7, in the vicinity of a disclination line 100, signal voltage does not carry out a controlling operation for achieving the even rotating directions of the liquid crystal molecules 98; therefore, roughness occurs and brightness is reduced on the screen of the liquid crystal display panel.